Lung cancer is the leading cause of cancer deaths worldwide. Patients have a dim prognosis with few treatment options. Thus, there is a critical need to develop molecular targets for lung cancer management. Compelling evidence indicates that mu () opioid receptors regulate non-small cell lung cancer (non- SCLC) growth and metastasis. Immunoblotting studies show over-expression of opioid receptors by the major histological types of non-SCLC cell lines, and by multiple human non-SCLC tumor samples. Silencing the receptors, or opioid receptor antagonist blockade, inhibits non-SCLC growth and metastasis in vivo in animal models. These laboratory studies were prompted, in part, by clinical observations of longer than anticipated survival of certain cancer patients receiving the opioid receptor antagonist methylnaltrexone. We now propose to identify metabolically stable [111In]-labeled tetrapeptide radioligands for SPECT imaging that bind selectively, and with high affinity, to the peripheral opioid receptors expressed by non-SCLC tumors. We will be able to leverage off experience gained during the development of hydrophilic [111In]- labeled naltrindole analogs that bind tenaciously to peripheral opioid receptors, including those expressed by SCLC tumors in mouse models. Diagnostic PET imaging has allowed visualization of opioid receptors on the primary tumors of four of four non-SCLC patients. The study establishes sufficient site density for successful human imaging. However, current radioligands for opioid receptor PET were designed for brain imaging, and are unfavorable for oncology studies in the periphery because of high lipophilicity. Specific Aims are: 1) to prepare [111In]-labeled tetrapeptides that bind with high affinity and selectivity to opioid receptors; 2) to characteriz opioid receptor binding in non-SCLC cells and tumor membranes using known [3H]-labeled radioligands (, , ?), as well as the novel [111In]-labeled opioid receptor tetrapeptides; and 3 to evaluate the pharmacokinetics, pharmacology, stability and SPECT imaging potential of the novel [111In]-labeled tetrapeptides in vivo in normal mouse, and in SCID mouse models of non-SCLC. Radioligands optimized for diagnostic SPECT studies of peripheral opioid receptors would be candidates for clinical translation. They have potential for stratification of non-SCLC patients into a cohort that might benefit from the emerging opioid receptor antagonist therapies.